Unravelling the geochemical and geochronological diversities of the pre-collisional magmatism

Author(s): Matee Ullah, Urs Klötzli, Christian Rentenberger, Jiří Sláma, Muhammad Younas, Muhammad Khubab, Mohammad Goudarzi, Tanveer Ahmad
Abstract: The magmatic arcs in the north-west region of Pakistan comprises of numerous volcanic and plutonic bodies of different ages and compositions evolved during the subduction of the Neo- Tethys Ocean under the Eurasian supercontinent. This study focusses on the examination of the granitoids of the Kohistan batholith (a part of Kohistan-Ladakh Island Arc; KLIA) and the Khunjerab pluton, concentrating on their petrological traits, mineral chemistry, in-situ zircon U-Pb geochronology, and whole-rock major and trace element geochemistry. According to zircon U-Pb dating, the Kohistan batholith granitoid was emplaced around 91.7 ± 0.3 Ma, while zircons of the Khunjerab pluton yield ages of 106.4 ± 0.4 Ma and 106.4 ± 1.0 Ma. All the samples from both magmatic units have calcic to calc-alkaline (Na₂O + K₂O: 3.6–10.6 wt.% and SiO₂: 60–73 wt.%), metaluminous to peraluminous properties (Aluminum Saturation Index (ASI): 0.9–1.2). Notably, Nb, Ta, and Ti show depletion, while large ion lithophile elements like Cs, Rb, and K have been enriched. Additionally, we find that SiO₂ and P₂O₅ have a negative correlation while Rb and Th have a positive correlation, which confirm an I-type arc magmatism. Together with the published literature, TEM analysis, and thermal modelling, our zircon U-Pb results point to a period of continuous magmatic activity from the Late Jurassic to the Late Cretaceous (between 150 Ma and 91 Ma) in the Kohistan Island arc region while the Khunjerab pluton (part of Karakorum block/Eurasian plate) experienced widespread magmatism around 120 Ma to 106 Ma. With SiO₂ concentrations ranging from 67.5–73.3 wt.% and 60–71.4 wt.% and relatively low alkali (Na₂O + K₂O) contents between 3.6–10.6 wt.% and 5.1–7.4 wt.% in the Kohistan batholith and Khunjerab pluton respectively, showing clear signs of acidity. The whole rock as well as the mineral geochemical analysis and the elevated water contents (8–10 wt.% and 3.1–3.5 wt.%) inferred from amphibole and biotite chemistry respectively, indicates that the Kohistan batholith was most likely formed through partial melting of a (hydrous) magma originating from a more or less altered metasomatized mantle wedge. Likewise, the Khunjerab pluton whole rock geochemistry also indicates its origin through partial melting of magma originating from an altered metasomatized mantle wedge. This study also shows that both units are not only different in terms of the nature of magmatism but also in terms of their ages i.e., continental arc magmatism occurred in the Khunjerab (Karakoram) block in the middle Cretaceous (106 Ma) while island arc magmatism occurred on the Kohistan side in the late Cretaceous (91 Ma). Further, this study also investigate why multi-grain U-Pb zircon dating is necessary for studying magmatic rocks by using transmission electron microscopy.
Organisation(s): Department of Lithospheric Research, Department of Geology, Physics of Nanostructured Materials
External organisation(s): Khushal Khan Khattak University, Czech Academy of Sciences, University of Houston, Lorestan University, Karlsruher Institut für Technologie
Journal: Geoscience Frontiers
Volume: 16
ISSN: 1674-9871
DOI: https://doi.org/10.1016/j.gsf.2025.102003
Publication date: 03-2025
Peer reviewed: Yes
Austrian Fields of Science 2012: 105127 Geochronology, 105101 General geology, 105105 Geochemistry
Keywords
ASJC Scopus subject areas: General Earth and Planetary Sciences
Portal url: https://ucrisportal.univie.ac.at/en/publications/011a5a64-327c-4485-83d5-747e0a7c2b1a